Typically in a building structure the loads from the building contents are supported by floor(s). Each floor in turn transfers the load to the beams or walls on which it is supported. The loads from the beams are transferred to columns and thereby to foundations. In some cases the walls directly transfer the load to the foundation underneath. A building may consists of several stories and many bays. Typically the loads from a bay are directly transferred to beams or walls and do not or only slightly affect the neighboring bays.
The floors are typically constructed out of wood, concrete, steel or combinations thereof For a wooden floor the wooden planks are typically supported on wooden joists like in the residential homes. A concrete floor typically includes a concrete slab having reinforcing rods embedded therein. In a composite floor system, the corrugated steel deck is supported by the joists and the corrugated steel deck supports the concrete slab. Prefabricated, prestressed, double "T" girder sections are also used as a floor system in industrial and commercial buildings. A floor refers to the combination of load carrying members such as the joists, deck and slab.
Besides carrying the stationary loads, floors also support dynamic loads such as machinery in operation in manufacturing plants, pedestrians in residential and commercial buildings, moving objects such as fork lifts on plant floors and vehicles on parking ramps, dancers in ballrooms, and exercisers in gymnasiums. In some cases a fluid flowing through a pipe or air flowing through a duct may be a source of a dynamic load. At times the floors are constructed using lighter joists and thinner floor slabs which makes the structure flexible. Additionally the span of each bay may be longer for cost advantages and/or multipurpose functions. This results in flexible floor systems with low inherent damping. In such situations the natural resonance of the floor may be excited, due to dynamic loads, leading to floor serviceability problems. The occupants may feel the vibrations and become annoyed or disturbed. In some cases the floor may support precision equipment, e.g., microscopes, where the serviceability problem may be severe. In some instances the building may be unsafe because of the floor vibration. It has been a challenge for the design engineers to either retrofit or design the floor to minimize the vibration problem.
Attempts which have been made in the past to reduce these floor vibrations may be classified into three categories.
First, vibration has been reduced by stiffening the floor using joists, steel and concrete beams.
Second, tuned mass dampers have been used to reduce floor vibrations. A tuned mass damper is a spring, mass and damping element system which is typically attached to the vibrating floor. The resonant frequency of the tuned mass damper is approximately the same as that of the floor. When the floor vibrates, the tuned mass damper is also excited and produces a counteracting force to the floor. The vibrational energy is dissipated through the motion in the damping device of the tuned mass damper. Hosono et al, Japanese patent No. 41992!-19344B, discloses a tuned mass damper for floor vibration damping. The damper includes an added mass and a spring with damping properties. The natural frequency of the damper is tuned to the natural frequency of the floor slab. The damper can be hung underneath the floor or set upon the floor.
Third, energy dissipating dampers have been used to reduce floor vibrations. Energy dissipating dampers are conventionally used in structures such as buildings, bridges, water towers, etc., to reduce the effect of vibration due to wind, earthquakes, etc. However, these are also used for reducing the vertical vibrations such as in the structural floors.
Yamanaka, Japanese Patent No. HO-6049924A, discloses a pair of support braces of which one end of one brace is connected to a column while one end of the other brace is connected to a floor. The pair of the support braces is made up of two components joined by a rotational cup and ball joint. Viscoelastic material is used between the cup and ball. Floor movement causes relative rotation between the cup and ball resulting in energy dissipation in the viscoelastic material. Since the rotation between the cup and ball is very small, this damper will not be efficient in dissipating the floor vibration.
Ikehara et al, Japanese patent No. 8-68132A, discloses a damper that consists of an external pipe raised straight from the lower structural floor and an internal pipe hangs from the structural floor above. Between the two pipes is a gap filled with the viscous fluid or viscoelastic material. This damper can attenuate the sound and vibration in both upper and lower floors. One drawback of this type of damper is that it can also transmit vibration between the two floors.
Yamanaka, Japanese Patent No. H06-49923A, discloses a damper having a lower steel tube and upper steel tube forming a gap between the tubes filled with a viscoelastic material. The tubes are attached straight up and down between floors to reduce the floor vibration. Again, one drawback of this type of damper is that it can also transmit vibration between the two floors.
Murray, U.S. Pat. No. 4,615,157, discloses a frictional damper for damping the joist of a floor spanning two supports. The frictional damper has a pair of overlapping plates. One of the plates is secured to the joist to be damped and the other plate is secured directly or indirectly to the ground. The friction between the overlapping plates acts to damp any oscillations in the joist. A friction material may be sandwiched between the plates.
Constrained layer damping is discussed by Fedric Nelson in "The Use of Visco Elastic Material to Damp Vibration in Buildings and Large Structure", AISC Engineering Journal, April 1968. In this paper a method to apply a constrained layer damping to a floor joist is proposed.
A brace viscoelastic damper in which the viscoelastic damper is installed inside the truss joist of a brace is disclosed by Tso Chein Pan, "Vibration of Pedestrian Overpass" Journal of Performance of Constructed Facilities, Vol 6, No. 1, February 1992.